The present invention relates to a distance measuring device for projecting a laser beam to an object to be measured and for measuring a distance by receiving a reflection light from the object to be measured.
As a distance measuring device, a light wave distance measuring device is known, which measures a distance to an object to be measured by projecting a laser beam to the object to be measured and by using a reflected light from the object to be measured.
In the past, when the light wave distance measuring device is used, intensity of a laser beam is modulated at a fixed frequency. The laser beam is projected as a distance measuring light, and the reflected distance measuring light reflected by an object to be measured is received. A phase of intensity modulation of the reflected distance measuring light as received is compared with a phase of intensity modulation of an internal reference light obtained by an optical path for reference which is formed in the distance measuring device, and a distance to the object to be measured is measured from the phase difference.
In the distance measurement in the distance measuring device as described above, the change of the phase difference depending on the measured distance is utilized. The phase difference between the internal reference light and the reflected distance measuring light Δφ is expressed by the following equation 1:Δφ=4πfD/C   (Equation 1)where D represents the measured distance, f represents modulation frequency, and C represents light velocity.
The measured distance D can be determined by measuring the phase difference Δφ. Because an optical path length for reference is already known, correct distance can be obtained by correcting the measured distance thus obtained on the internal reference light optical path.
In the distance measurement, drift in detection circuit etc. inside the distance measuring device exerts influence as measurement error. By comparing the phase of the internal reference light and the reflected distance measuring light, the drift in the detection circuit etc. is offset, and correct distance can be calculated.
Referring to FIG. 12, description will be given below on general features of a conventional type distance measuring device.
A light emitting element 1 such as a laser diode emits a laser beam of which intensity is modulated to a predetermined frequency by a light emitting element driving circuit 12. By a half-mirror 2, the laser beam is split to a distance measuring light 3 and an internal reference light 4. The distance measuring light 3 transmits the half-mirror 2 and then the distance measuring light 3 is projected to an object 6 to be measured, e.g. a reflection mirror such as corner cube via an objective lens 5. A reflected distance measuring light 3′ as reflected by the object 6 to be measured passes through the objective lens 5 and a half-mirror 8 and the reflection distance measuring light 3′ is received by a photodetection element 7 such as an avalanche photo-diode.
The internal reference light 4 reflected by the half-mirror 2 is further reflected by the half-mirror 8 on an optical path of the reflected distance measuring light 3′, and the internal reference light 4 is received by the photodetection element 7. A photodetection signal of the photodetection element 7 is inputted to a photodetection circuit 13. The photodetection circuit 13 performs processing on the signal inputted from the photodetection element 7 for calculating the distance.
An optical path switcher 9 is provided to stretch over the optical path of the distance measuring light 3 and the optical path of the internal reference light 4. On the optical path of the reflected distance measuring light 3′, a light amount adjuster 11 is provided. The optical path switcher 9 selectively intercepts one of the optical path of the distance measuring light 3 and the optical path of the internal reference light 4 and transmits the other. The reflected distance measuring light 3′ and the internal reference light 4 are alternately received by the photodetection element 7.
As described above, the distance measuring light 3 of which light intensity is modulated is used, and the distance is calculated by obtaining phase difference between the internal reference light 4 and the reflected distance measuring light 3′ obtained from the distance measuring light 3. Due to difference of the receiving light amount (photodetection light amount) of the reflected distance measuring light 3′ and the internal reference light 4, phase error is caused by amplitude of the photodetection element 7 or a circuit. The phase error gives influence on the accuracy of the distance measurement. Therefore, the light amount adjuster 11 is disposed for the purpose of eliminating the difference of the receiving light amount. The light amount adjuster 11 comprises a density filter with the density continuously changed. By rotating the density filter, the receiving light amount of the reflected distance measuring light 3′ is adjusted to a constant level. By the light amount adjuster 11, it is so arranged that the receiving light amount of the internal reference light 4 received by the photodetection element 7 is to be equal to the receiving light amount of the reflected distance measuring light 3′, even if reflecting light amount is changed according to the distance of the object to be measured 6.
Optical path switching by the optical path switcher 9 and light amount adjustment by the light amount adjuster 11 are controlled by a driving circuit 14.
A control arithmetic unit 15 controls the light emitting element driving circuit 12 in such manner that a laser beam emitted from the light emitting element 1 has light intensity modulation of a predetermined frequency, and the control arithmetic unit 15 controls the timing of optical path switching of the optical path switcher 9 by the driving circuit 14. Further, the control arithmetic unit 15 sends out a control signal to the driving circuit 14, and the control signal equalizes the light amount of the reflected distance measuring light 3′ to the light amount of the internal reference light 4 based on the photodetection signal of the photodetection element 7.
The photodetection circuit 13 performs signal processing such as amplifying, A/D conversion, etc. on the signal from the photodetection element 7, at the same time, further performs the processing such as the determination etc. of phase difference between the modulated frequency of the internal reference light 4 and the modulated frequency of the reflected distance measuring light 3′. The photodetection circuit 13 sends out the signal from the photodetection element 7. Based on the phase difference sent from the photodetection circuit 13, the control arithmetic unit 15 calculates a distance to the object 6 to be measured from the above equation 1.
In the conventional type distance measuring device as described above, the switching between the internal reference light 4 and the reflected distance measuring light 3′ is mechanically switched over by the optical path switcher 9.
Because both the optical path switching and the light amount adjustment are carried out mechanically, it is difficult to perform optical switching at high speed or the light amount adjustment at high speed, and it is difficult to measure the distance at high speed. For this reason, there is no problem when distance is measured on an object to be measured such as a building. However, when distance is measured continuously on a plurality of moving objects such as a construction machine, e.g. a bulldozer etc., by a single measuring device, that is when it is necessary to measure the distance at high speed, the distance measurement is often difficult to carry out. Also, in case 3-dimmensional measurement is to be carried out on a building etc. by using a total station etc., surveying must be made on multiple points by automatic surveying, and the measurement must be performed at high speed. In case surveying is performed on a moving object, There were problems such that the speed of the optical path switching and the speed of the light amount adjustment often cannot follow the moving speed of the moving object, and that the distance measurement cannot be carried out.
As a distance measuring device to determine the distance at multiple points and multiple directions by rotating a distance measurement light, the devices described in the Japanese Patent Publication No. 2694647 or in the Japanese Patent Publication Laid-Open 4-313013 are known.